An interesting video that seems to present multiple theories.
It posits that Gaster made the prophecy as a means to free himself. Banish The Angel's Heaven, The Depths, & he gets freed of them. This happens at the Roaring, & Toby Fox's 1st Tweet, back in 2011, was "The edge of the shadow, where reality and dream meet.". Ergo, for Gaster to be able to enter into the Light World, rather than the Dark World that he's already shown influence over, it must happen during The Roaring, when The Light World & The Dark World(s) are in chaos & the latter threatens to overtake one another; When the fiction/dream-analogous Dark World(s) & the reality-analogous Light World meet.
Further, there is a demon called an angel, Lucifer, who failed to usurp God's power, failed, & was banished from heaven in an event called "Lucifer's fall from Heaven".
Gaster discovered Dark Worlds & "fell into his own creation".
Also, The Dark Fountain theme is called "THE_HOLY" in all caps, like how Gaster speaks, yet Gaster is renowned for his association with darkness, the number 666, & to some extent, demonic imagery. (Like that big metal goat skull thing in The True Lab in Undertale.)
This is evidence said theory uses to argue that Gaster seeks to trick/usurp us. He is in control of the Save Files, & will readily delete or copy any of them, despite ostensibly having little reason to care about or little care for timelines that can freely be rewound, restarted, deleted or copied; What matters is his end goal, which is the one ending of the player banishing The Angel's Heaven during The Roaring, so that he may be freed & get what he wants.
1 possibility the theory posits as an alternative to us battling is Gaster ejecting the player out to the title screen, but with their file overwritten & locked, now with the name GASTER, the name you cannot normally use for your SAVE file.
It also speculates that we may need to hone Noelle's glitches/secrets-finding abilities through the Weird Route to get the freedom we need so that we can free December Holiday, who can somehow help us overcome Gaster.
Or there's some crazy meta hunt.
Although, apparently the video maker has strong, somewhat embittered opinions about theories that put parts together like puzzle pieces & feels fans are often neglecting that sometimes stuff just has mistakes, or is meant to be comedic, or just conveys info, rather than being meant as part of some deeper meaning.